Wide angle lens?

Coming from m4/3, I recently bought a 6D, and have a 50mm; 70-200, and the Tamron 150-600 is on order. I'm not familiar with the offerings for wide angle lenses, so I'm seeking advice on what one lens would be a good choice for the 6D. I don't even know if I should get a prime or something with a little zoom. (General shooting/landscapes). Any advice?

Facing a similar dilemma (have a Sigma 10-20 but its an APS-C lens and will be getting sold). Although my 24-105L is now actually reasonably wide on FF. My shortlist that I'm looking into are..

- Sigma 12-24mm II (crazy wide at the short end and OK without being wow)

- Canon 17-40L (the safe choice and the only one of the three that takes filters)

- Sigma 15mm diagonal fisheye (which looks intriguing, very good quality and is the cheaper of the three)

The Samyang others mentioned I was initially very much drawn to but put off a bit that some of the distortion is very difficult to correct, which limits its application slightly. Seen some great images from it though

As for zoom versus prime, its the usual trade-offs. You gain versatility but can lose a little in IQ and sometimes distortion and speed. Given even a 10mm range of zoom, the difference in framing of shots at either end of that is massive.